You could try hanging the cilantro on the outside of the cage. Sometimes, they won't bother checking out any unfamiliar objects when you just put it in the cage. It's part of the instinct to be wary of things that seem unfamiliar to them. However, if it's on the outside of the cage, they feel safe enough to check it out. May take a few days, but they would eventually become curious, and it would still take less time than waiting for them to pick at it while it's inside the cage.

I looked up toxic plants for parrots. The wood, as well as the fruit, is considered safe. Not sure about the leaves though, since they didn't mention it at all. Might be best to take Carolyn's advice and not let her eat too much of them, just in case.

why cilantro?
I can foresee, someone will ask: why not?

Cockatiels tend to like veggies with thin stems so cilantro is more attractive to them than some veggies. I'm surprised that Coco didn't go for it, since it's frequently on the menu here. Maybe it's because he isn't used to seeing it in his new home, but if you try again from time to time he'll probably end up eating it.